ID1 Is A Pro-Survival Gene Responsible For Glioblastoma Initiation And Tumor Growth

Rohit SACHDEVA, St. Michaelís Hospital, Canada
WU M. 1 , CELEBRE A. 1 , CHAN J. 1 , GUAN J. 1 , DAS S. 1,2

1 Department of Neurosurgery, St. Michaelís Hospital, Toronto, Canada
2 Brain Tumor Research Center, Sick Kids Hospital, Toronto, Canada

Glioblastoma multiform (GBM) is one of the most aggressive primary brain tumors. Progress in the treatment of GBM over the last few decades has remained marginal and GBM is still universally fatal with short survival times after initial diagnosis. Temozolomide (TMZ) was the last drug to bring a significant improvement of survival for patients with GBM. Resistance to TMZ is largely due to glioblastoma stem cells (GSC) population.
 
Transcription regulatory protein, inhibitor of DNA Binding 1 (ID1), has been implicated as a key regulator in GSC. Studies have shown an increase in ID1 expression in a variety of solid tumors. Up-regulation of ID1 correlates with both poor prognosis and chemo resistance. In our study, we investigated the role of ID1 in response to TMZ chemotherapy in GSCs.
 
We were able to show that ID1 protein expression is up-regulated in response to TMZ in GCS cell lines. We found a majority of GSCs are resistant to TMZ and are able continue proliferating post-TMZ treatment. The next step was to use an effective method for investigating ID1 loss of function and its influence on tumorogensis. Here we utilized CRISPR/Cas9-mediated somatic gene disruption.  We established a knockout of ID1 in immortalized GSC cell line U251.  Post-TMZ treatment, ID1-/U251 cells displayed significantly lower instance of relapse in cell growth. This result indicated in-vitro ID1 acts as a pro-survival protein in GBM post chemotherapy.
 
We then exploited this finding by injecting ID1-/-U251 cells along with a luciferase reporter into a mouse xenograft model which resulted in a very substantial delay in tumor formation compared to control, thereby leading to extended survival times. Hence suggesting ID1 is a key factor responsible for GBM initiation and tumor growth. Combined our results provide a rationale for ID1 inhibition as a synergistic therapeutic approach along with chemotherapy against GBM.